1. Technical Field
The present invention relates generally to a system for controlling the steering angle of the rear wheels of a vehicle. More particularly, the invention relates to a system for controlling the steering angle of the rear wheels in view of a parameter with respect to suspension steer.
2. Background Art
In an automotive vehicle without rear wheel steering (2WS), yaw rate gain and phase lag corresponding to the steering frequency f of the steering wheel tend to be quite low and to become greater as the steering frequency increases. Four wheel steering (4WS) vehicles have accordingly been developed. A system for steering the rear wheels of such a 4WS vehicle is adapted for controlling the central yaw position using the two-degrees-of-freedom model in view only of yawing and lateral displacement of the vehicle. Thus, based on a control transfer function for steering the rear wheels expressed theoretically by one linear expression over another linear expression (i.e. X/x), a target rear wheel steering angle is determined.
It will be appreciated that the prior art system does not take account of variations in attitude during running such as roll and compliance steering or suspension steer caused by lateral forces acting on a wheel when a slip angle is produced on the wheel. As a result, due to roll steering (the influence of compliance steering is relatively small), the value of the rear wheel steering angle does not increase, causing variation in yaw rate and phase lag to increase. This results in only a small improvement in yaw damping from the rear wheel steering.
It is a desired characteristic that the value of the steering angle increase as the steering frequency becomes higher.